If I had two million, I’m a two-millionaire.
If I had one million, I’m a one-millionaire.
If I had a million, am I an a-millionaire?
On a slightly similar topic: https://www.grammarly.com/blog/hyphen-in-compound-adjective-with-numbers/ we know that we can make compound adjectives by hyphenating numbers.
The president gave 10 one-minute speeches. This is valid.
Can you correctly say He gave 10 a-minute speeches?
My class is divided over this issue. Half is defending that "I am an a millionaire" is grammatically correct, despite it being obviously confusing and frowned upon. The other half thinks there is no hard rule that allows a to be interchanged with one that would warrant having the two articles, an a next to eachother.